Means for producing focused high density electron streams



Jan. 18, 1966 w. H. BENNETT MEANS FOR PRODUCING FOCUSED HIGH DENSITY ELECTRON STREAMS 2 Sheets-Sheet 1 Filed Oct. 30, 1961 T i Gmwzw zoEQwd 000 09 000 9 000; 00. o. No mo H/ lr N. f /N l I N (amssaxdfiuw Jadf (mod) L110 19 SNOILVZINOI ATTORNEY Jan. 18, 1966' Filed Oct. 30, 1961 L5 mfd IOOK .n

w. H. BENNETT 3,230,419

MEANS FOR PRODUCING FOGUSED HIGH DENSITY ELECTRON STREAMS 2 Sheets-Sheet 2 2 N OKID 4- mm D l I 4 \l/ INVENTOR W\LLARD H. BENNETT ATTORNEY United States Patent O 3,230,419 MEANS FOR PRODUCING FOCUSED HIGH DENSITY ELECTRON STREAMS Willard H. Bennett, 5032 Kaplan Drive, Raleigh, N.C. Filed Oct. 30, 1961, Ser. No. 148,776 3 Claims. (Cl. 315.-111)- (Granted under Title 35, US. Code (1952), see. 266) The invention described herein may be manufactured and used by or.- for the Government of the United States of America for governmentalv purposes without the payment of any royalties.

This invention relatesin general to electron beams or streams and more particularly tohigh current streams of small-diameter.

In the field of electron-beam production andfocusing it has become increasingly desirable to'produce beams or streams of larger and larger current carrying capability yet maintaining small cross-sectional. areas. Among the difiiculties yet tobe overcome in the production'of such streams is that of reducingor avoiding the etfect of mutual electrostatic repulsion of the electrons in the stream which results generally in a minimum diameter of stream for any. combination of current and voltage. Further, the mini+ mum diameter has heretofore been maintainable only along a short length of stream. beyond which the beam blows up.

Prior attempts to overcome or reduce electron stream blow-up due to space charge, such as neutralizing the electron space charge with positive ions, have been only partially successful.v One such attempt, that of gas focusing in which the high energy electrons-ionize residual gas atoms or molecules, with the electrons displaced from the molecules during ionization departing laterally from the stream. with energies of the order of 1-0 electron volts, leaves the positive ions behind in the stream of high energy electrons. This action limits the maximum positive space k small-diameter streams hasbeen the development and use of such streams in velocity modulated micro-wave tubes and in related applications. Highly selective electrode shaping between the cathode and anode for the purpose of driving electrons from the cathode together into a concentrated beam of small diameter has not prevented electrons in the beam from flying apart again because of mutual electric repulsion after the beam or stream has been driven for a short distance. Magnetic or electric focusing lenses can be used to keep the stream somewhat concentrated .but as soon as the stream passes the last lens in the train the stream quickly flies apart. The use of lenses only delays the blow-up, it does not eliminate blow-up.

Again referring to attempts to control blow-up through neutralization of the space charge with positive ions, this method has been considered impractical primarily because the ions were those resulting from the ionization of the very low density residual gas in a tube, and residual gas cannot readily be maintained at a stable pressure in a sealed tube. Consequently, the rate of production of ions varies uncontrollably in a sealed tube. Further, the residual gases usually encountered contain oxygen and carbon, and the ions of both of these are cathode poisons. Positive ions of these substances formed in front of the cathode are attracted to the cathode, causing deterioration of the cathode due to the ion bombardment. Other spurious effects have been observed; however the effects have been irregular and not well understood.

The present invention provides a method and means for obtaining large electron currents in small-diameter streams by space charge neutralization which will produce a stable stream in ionized media and yet avoid the disadvantages of prior methods and devices.

Accordingly, it is an object of the present invention to provide means to producean electron stream of large current and small diameter.

Itis another object of thisinvention to provide means to produce an electron stream of large current and small diameter which is substantially stable in an ionized media.

It is a further o'bjectof the. present invention to provide means for. producing an electron stream of highdensity by space charge neutralization.

It is a further object of this inventionto provide means for producing an electron stream of large current and small diameter whichis self-focused.

Other objects and advantages of this invention will become apparent on a careful consideration of the following description when read in conjunction with the accompanying drawings in which like reference numerals designate like parts throughout the figures thereof and wherein:

FIG. 1 is a sectional view of one embodiment of the invention.

FIG. 2 is a circuit diagram of components used in con nection with the embodiment of FIG. 1.

FIG. 3 is a graph illustrating the efiiciency of hydrogen ionization.

Referring to FIG. 1, there is shown the tube 11 of one embodiment of the invention which includes glass envelope 12 having an-inlet at 13.where hydrogen or another easily ionizable gas may be admitted, openings at 15 and 16 where pressure is measured, and an outlet at 17 through which the gas entering at 13 is exited by means of a pump, not shown. Cathode 1 is a source of electrons which are drawn toward. anode 20 while a potential, negative relative to the cathode, is applied to electrode 23. Tube. 11 is pumped at outlet 17 while hydrogen gas is admitted at inlet 13 at a rate sufficient to maintain a pressure of hydrogen at opening 15 of substantially 10 to l0 millimeters of mercury. The pressure at opening 16 will be less than that at opening 17 depending upon the size of aperture 25 in anode 20. Electrodes 30, 31 and 32 are positioned to collectelectrons which have become detached from electron stream 35. Current measuring means, not shown, may be included to measure the collected current at each of the electrodes 30, 31 and 32; the current being produced by detached electrons following representative paths 36, 37 and 38, respectively.

Shown in FIG. 2 andfurther explained in my copending application, Serial No. 190,513 filed April 26, 1962, is one embodiment of a circuit capable of supplying desired current tothe tube of FIG. 1. The circuit of FIG. 2 is used for delivering well-regulated square-topped one millisecond pulses of 25 kv., negative, to cathode 19 at any repetition frequency up to one per second. Longer pulses may be obtained 'by increasing condenser capacity.

FIG. 3 is a graph indicating the efiiciency at which the electrons ionize hydrogen. It can be seen that at 25 kv. ionization efficiency is 0.07 ionization per centimeter path and per millimeter of pressure. A 25 kv. electron travels at 0.3 times the velocity of light or 9x10 centimeters per second, and in hydrogen at 5X10 millimeters of mercury pressure, each electron on an average produces one ionization each 32 microseconds.

In operation, the electron stream 35 delivered from cathode 19 converges toward a point 21 between cathode 19 and anode 20 and then spreads out again in the form of a cone indicated by lines 36. The electrons ionize the hydrogen with an efiiciency that can be obtained from a curve in FIG. 3; namely, for the conditions specified, one ionization by each electron every 32 microseconds. The electrons at an energy of 25 kv. lose in the order of 30 electron volts of energy per ionization of which about 15 electron volts goes toward ionizing the hydrogen and most of the remainder toward giving transverse momentum to the resulting free electrons which are detached from the gas molecules. The beam electrons lose negligible momentum in these collisions and continue in the stream towards electrodes 31 and 32. The stream continues to lose the detached electrons until it attains a potential of about volts, positive, inside electrode 30 and beyond. The cone of beam electrons is reduced from that indicated by 36 to that indicated by 37 in a little more than 32 microseconds through neutralization of space charge.

After the stream has flowed long enough for ionization of residual gas to neutralize space charge, the stream partially collimates; however, it still disperses in a cone represented by 36, 37 and 38. By adjusting the potential on electrode 23, the beam can be made to move a short distance beyond anode in sufiiciently parallel form to satisfy the condition for self-focusing, and when this is accomplished the stream collimates progressively inside electrode 31.

Neutralization of space charge draws the beam down to a cone represented by 37 in a little more than 32 microseconds, however, this does not completely focus the stream because of the transverse components of mo mentum of high energy electrons, mostly due to imperfections in the collimating elfect and space charge in the throat of anode 20. Although the stream space charge becomes slightly over-neutralized in the otherwise equipotential region in electrode and beyond, the positive ions are strongly accelerated from anode 20 toward cathode 19, and the space charge does not ever become fully neutralized between the cathode and anode. Thus, by adjusting the potential of electrode 23 relative to cathode 19, and adjusting the distance between the cathode and anode, the space charge between the cathode and anode can aid in collimating the electrons into a straight parallel section at 21.

It is necessary to satisfy the conditions for magnetic self-focusing, or pinch eifect, in order to produce a fully focused stream from the spreading cone indicated at 37. Such conditions are that in a stream Whose space charge is neutralized, the total current I in the stream must exceed where N is the linear particle density, C is the speed of light, and 9 is the average energy due to momenta transverse to the direction of the stream. 6 is determined by the equation:

6=W tan a where W is the energy of an electron in the stream due to momentum in the direction of the stream and Where a is the root mean square angle between the direction of motion of the stream electrons and the axis of the stream. For self-focusing, a must be smaller than given by:

and for a 3-ampere stream of 25 kv. electrons, a must be smaller than 1.3". Such a desirable degree of collimation is obtainable with careful positioning of anode 20 and careful selection of the potential applied to electrode 23.

When the self-focusing conditions are satisfied at 21, the pinch will be propagated along the stream and a fully focused stream will result. The speed of propagation of the pinch can be obtained by calculation though with difiiculty, or measured by observing the collected current at each of the electrodes 30, 31 and 32 during the one millisecond that potential is applied to the cathode.

There is thus provided means for producing a collimated electron stream of high density by space charge neutralization wherein the stream is self-focused magnetically and random transverse components of velocity, i.e., dispersal, is overcome by effective self-focusing. The

total current in the stream is greater than the minimum critical current for self-focusing, i.e., the minimum current is proportional to the average energy of the stream electrons due to components of momentum transverse to the direction of the stream. The stream provided by the present invention overcomes the stream blow-up due to space charge which is encountered in other present and prior devices.

Many modifications and variations of the present invention are possible pursuant to the above teachings. It is therefore understood that its practice is not to be limited by the specific examples in the foregoing examples in the foregoing description and that this invention is only to be limited by the scope of the appended claims.

What is claimed is:

1. A device for producing high density electron streams which comprises a cylindrical envelope having a longitudinal axis, a cathode positioned at one end of said envelope about the axis, an anode spaced linearly a short distance from said cathode along the axis, an axially aligned aperture through said anode, said cathode and anode adapted to provide a flow of electrons emitted by said cathode through said aperture along the axis of said envelope, an electrode positioned outwardly of said cathode about the axis of said envelope and extending toward said anode about the space between said cathode and said anode, said electrode adapted to aid in collimating electrons emitted by said cathode and directing said electrons through said aperture in said anode, a plurality of axially aligned electrodes spaced along the axis of said envelope downstream from said cathode and anode, said electrodes adapted to collect electrons detached from said electron stream, and an inlet at one end of said envelope and an outlet in the opposite end thereof for permitting a flow of ionizable gas through said envelope along said electrodes, anode and cathode.

2. A device for producing high density electron streams which comprises an elongated cylindrical glass envelope having a longitudinal axis, a cathode and an anode, said cathode being positioned near one end of said envelope in spaced axial alignment with said anode with a concave face facing said anode, an electron focusing electrode positioned outwardly of said cathode and extending toward said anode about the spacing between said cathode and said anode, an axially aligned aperture in said anode, said focusing electrode and said cathode adapted to focus electrons emitted by said cathode through said aperture in said anode and beyond, a first electrode positioned adjacent to said anode and axially aligned therewith, said first electrode extending axially away from said anode and including a surface perpendicular to said axis, said surface perpendicular to said axis including an axial aperture therein, a second axially extending electrode positioned in axial alignment with said first axially extending electrode, said second electrode having an open end adjacent said face of said first electrode, said second electrode extending linearly about said envelope axis over a length substantially greater than said first electrode, said second electrode including a face near the end thereof remote from said first electrode which is perpendicular to said axis, said face on said second electrode having an axial aperture therein, said aperture in said second electrode being of lesser diameter than said aperture in said first electrode and passes a beam of electrons therethrough, a third electrode positioned adjacent to said second electrode and extending axially within said envelope, said first and second electrodes adapted to collect detached electrons of said electron stream and to pass a pencil beam of electrons through the aperture in their faces, said third electrode adapted to collect detached electrons of said electron stream that passes into the end thereof adjacent to said second electrode, and an inlet and an outlet in said envelope for permitting a flow of ionizable gas through said envelope along said electrodes, anode and cathode.

3. A system for producing high density electron streams which comprises a cylindrical envelope having a longitudinal axis, an inlet port at one end of said envelope, an outlet port at the opposite end, and a plurality of axially spaced electrodes assembled within said envelope, said plurality of electrodes including an electron focusing cathode, an anode axially spaced from said cathode, said anode including an axial aperture therein through which electrons emitted by said cathode are focused, an electron focusing electrode positioned outwardly of said cathode and extending axially of said envelope toward said anode, first and second axially spaced electrodes in axial alignment with said anode, each of said first and second electrodes having an open end and a face on one end with an aperture in said face with said open end facing said anode, and a third electrode axially spaced from said second electrode, an electrical circuit means electrically connected with said electrodes for producing pulses of electrons, said electrical circuit applying square topped pulses at a potential of about 25 kilovolts to said electron focusing electrode and a slightly more positive pulse respectively to each the cathode, anode, and first, second and third electrodes, said inlet and outlet permitting a flow of ionizable gas through said envelope along said electrodes, said electrodes adapted to collimate electrons emitted by said cathode on passing through said envelope wherein the electrons are held in an axial path by magnetic self-focusing due to electron flow and said first, second, and third electrodes collect electrons detached by ionization.

References Cited by the Examiner UNITED STATES PATENTS 2,817,036 12/1957 Neal 313--231 2,883,568 4/1959 Beam et a1. 313-7 GEORGE N. WESTBY, Primary Examiner.

RALPH G. NILSON, Examiner.

W. F. LINDQUIST, S. SCHLOSSER,

Assistant Examiners. 

1. A DEVICE FOR PRODUCING HIGH DENSITY ELECTRON STREAMS WHICH COMPRISES A CYLINDRICAL ENVELOPE HAVING A LONGITUDINAL AXIS, A CATHODE POSITIONED AT ONE END OF SAID ENVELOPE ABOUT THE AXIS, AN ANODE SPACED LINEARLY A SHORT DISTANCE FROM SAID CATHODE ALONG THE AXIS, AN AXIALLY ALIGNED APERTURE THROUGH SAID ANODE, SAID CATHODE AND ANODE ADAPTED TO PROVIDE A FLOW OF ELECTRONS EMITTED BY SAID CATHODE THROUGH SAID APERTURE ALONG THE AXIS OF SAID ENVELOPE, AN ELECTRODE POSITIONED OUTWARDLY OF SAID CATHODE ABOUT THE AXIS OF SAID ENVELOPE AND EXTENDING TOWARD SAID ANODE ABOUT THE SPACE BETWEEN SAID CATHODE AND SAID ANODE, SAID ELECTRODE ADAPTED TO SAID IN COLLIMATING ELECTRONS EMITTED BY SAID CATHODE AND DIRECTING SAID ELECTRONS THROUGH SAID APERTURE IN SAID ANODE, A PLURALITY OF AXIALLY ALIGNED ELECTRODES SPACED ALONG THE AXIS OF SAID ENVELOPE DOWNSTREAM FROM SAID CATHODE AND ANODE, SAID ELECTRODES ADAPTED TO COLLECT ELECTRONS DETACHED FROM SAID ELECTRON STREAM, AND AN INLET AT ONE END OF SAID ENVELOPE AND AN OUTLET IN THE OPPOSITE END THEREOF FOR PERMITTING A FLOW OF IONIZABLE GAS THROUGH SAID ENVELOPE ALONG SAID ELECTRODES, ANODE AND CATHODE. 